The present invention generally relates to image transfer technology and, more particularly, to an apparatus for forming a latent electrostatic image on an imaging surface, and an image transfer device utilizing the apparatus.
As used herein, the term “image transfer device” generally refers to all types of devices used for creating and/or transferring an image in an electrostatic imaging process (also referred to as ion deposition printing, charge deposition printing, ionography, electron beam imaging, and digital lithography, for example). Such image transfer devices may include, for example, laser printers, copiers, facsimiles, and the like.
In an image transfer device using electrostatic imaging, an electrostatic latent image is formed on a dielectric imaging surface by directing beams of charged particles onto an imaging surface. The electrostatic latent image thus formed is developed into a visible image using electrostatic toners or pigments. The toners are selectively attracted to the electrostatic latent image on the imaging surface, depending on the relative electrostatic charges of the imaging surface and toner. The imaging surface may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. A sheet of paper or other medium is passed close to the imaging surface (which may be in the form of a rotating drum or belt, for example) thereby transferring the toner from the imaging surface onto the paper, thereby forming a hard image. The transfer of the toner may be an electrostatic transfer, as when the sheet has an electric charge opposite that of the toner, or may be a heat transfer, as when a heated transfer roller is used, or a combination of electrostatic and heat transfer. In some imaging system embodiments, the toner may first be transferred from the imaging surface to an intermediate transfer medium, and then from the intermediate transfer medium to a sheet of paper.
The source of the beams of charged particles in an image transfer device using electrostatic imaging is referred to as a charge deposition print head, or simply “print head.” The present invention relates to charge deposition print heads of the type wherein selectively controlled electrodes, generally arranged in two or more layers separated by insulating layers, are disposed to define a matrix array of charge generators from which charge carriers are directed at the imaging surface moving along a scan direction past the print head. Such charge deposition print heads allow the matrix of charge generators to form an image of arbitrary length, with high resolution, on the imaging surface as it moves past the print head.
In such charge deposition print heads, generator electrodes on a first side of the insulating layer are activated with an RF signal of up to several thousand volts amplitude, while lesser bias or control voltages are applied to discharge electrodes (sometimes referred to as finger electrodes) on the opposite side of the insulating layer to create localized charge source regions located at or near crossing points between the generator and discharge electrodes. Specifically, the discharge electrodes include apertures at which electrical air gap breakdown between the discharge electrode and the insulator causes generation of electrical charge carriers. The charge carriers escape from the apertures and are accelerated to the imaging surface where the charge is deposited. The print heads may be configured to deposit either positive or negative charge, and the negative charge may consist partly or entirely of either ions or electrons. The print heads are configured so that the charge deposited by each aperture forms a pixel or dot-like latent charge image on the imaging surface as it moves past the print head. Each raster scan of the print head electrodes thus fills a narrow image strip, with the totality of image strips forming an image page.
Observation of the onset of charge particle generation in prior art print heads shows that the voltage required to initiate charge particle generation varies between aperture sites. This results in non-uniform charge particle output between aperture sites, and corresponding non-uniformity in the pixels forming the electrostatic latent image on the imaging surface. Further, at diameters smaller than about 100 microns, the discharge voltage rises and non-uniformity effects become severe. For these and other reasons, prior art charge deposition print heads use a discharge electrode aperture diameter of about 150 microns, thus limiting the capability of printing high resolution or light tones.